1. Field of the Invention
The present invention relates to an image formation method.
2. Discussion of the Background
In the electrophotographic image forming apparatus such as a printer or a photocopier, a two component developing agent has been suitably used in light of image quality, durability and high speed performance. In such a two component development system, a method is taken in which a magnetic brush of a developing agent contacts with the surface of an image bearing member and the perimeter speed of the development sleeve increases relative to the perimeter speed of the image bearing member to secure the image density and improve the fine line reproducibility.
On the other hand, with the advance of full colorization, systemization and digitization, the demand for improvement on image quality, performance speed and stability have been strong in the electrophotography industry of late. In this situation, it is expected to penetrate into quick printing industry with photocopiers and various kinds of printers which can satisfy such a demand. To march into the printing market using the technology of electrophotography for use in photocopiers and various kinds of printers, it is required to maintain and secure production of quality images with a high stability at a high speed processing for an extended period of time.
In the image forming apparatus capable of producing quality images up to the standard in the printing market by rotating an image bearing member and a development sleeve, the amount of charge of a developing agent should be imparted instantly and stably maintained. Thus, in such a situation, the damage to the developing agent in the development device, specifically, the damage caused by abrasion between toner and charging member and toner and carrier, is significantly different from the case of an image forming apparatus rotating at a low or middle range speed. Due to this damage, the external additive attached to the surface of toner is embedded or buried therein so that the toner particles tend to agglomerate. Furthermore, in an image forming apparatus for use in the high speed rotation range described above, the temperature inside the apparatus easily rises due to such continuous image output for an extended period of time in comparison with an image forming apparatus for use in a low or middle rotation speed range. This also applies to the development device, which promotes agglomeration of a developing agent (toner). In addition, this agglomeration tendency is promoted by the size increase of an image forming apparatus made to deal with high speed performance, especially in the transfer path of a developing agent which is resultantly long.
In an image forming apparatus outputting images at a super high speed in which the abrasion damage between the developing agent and the charging member is large and the temperature tends to rise, part of the developing agent which has been damaged by abrasion and/or has a tendency to agglomerate in such a high temperature environment passes through the charging member in agglomeration. This results in whiteout phenomenon in which the image portion corresponding to the agglomerated developing agent is missing. Quality images are not stably output when such a phenomenon occurs.
To solve this problem, unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2000-305310 describes a technology in which, in addition to a fluidizer, a resin particulate agglomeration body which is re-dispersed upon a mechanical stress is attached to the surface of toner particles. After the fluidizer is embedded or buried in toner particles, the re-dispersed resin particulates cover the surface thereof to prevent the agglomeration of the toner particles. However, both the resin particulates and the fluidizer are embedded or buried in the toner particles over an extended period of time. Thus, this technology finally depends on the agglomeration force mother toner particles have before the fluidizer and the resin particulates are attached thereto.
In JOP 2006-308849, toner agglomeration in a development device is restrained by controlling the environment (temperature and humidity) in an apparatus.
With regard to the binder resin for use in a toner, to obtain fine gloss, excellent color development and a wide fixing temperature range for a full color image, JOP S61-007844 describes a polyester resin and JOP 2003-173045 describes a polyol resin. However, when the polyester resin is used, toner tends to agglomerate, especially in the toner bottle or the development device in an image forming apparatus outputting images at a super high speed, resulting in occurrence of whiteout. When the polyol resin is used, toner agglomeration can be relatively prevented in comparison with the case in which the polyester resin is used. However, due to the existence of hydroxyl group in the polymer chain in the polyol resin, the toner slightly has a moisture absorption characteristic, which makes toner particles agglomerate in a development device. This leads to occurrence of whiteout in some cases. Therefore, the objective of stably producing quality images is not achieved.